1. Field of the Invention
The present invention relates generally to electrical transformers, and more particularly relates to high voltage transformers having a substantially high resistivity magnetic core material.
2. Description of the Related Art
Traditionally, in order to derive a high voltage DC output from a relatively lower voltage, one would utilize a transformer having a high secondary to primary turns ratio. Typically, for a fixed wind layer patterning and overall geometry, a second parasitic capacitive impedance reflected to the primary decreases linearly with frequency as well as the square of the turns ratio. Therefore, as the turns ratio, i.e., required output voltage, increases for a fixed output power level and optimum operating frequency, the reactive current that is required to drive the secondary parasitic capacitance becomes greater and may surpass the non-reactive current responsible for power flow to an output rectifier. In order to compensate and limit converter and transformer copper losses, increasing turn ratios is typically not used as a means to achieve high voltage DC output.
In the 1930s Cockcroft and Walton first demonstrated a now-common method of achieving high voltage DC output. Such method is still widely used in industry at power levels mostly below about 50 kW.
Cockcroft and Walton developed a voltage multiplier that provides capacitive current transfer to rectifier stages which operate at potentials defined by the outputs of lower capacitively coupled rectifier stages. Operation of such voltage multiplier is sustained by virtue of the coupling capacitors receiving a charging current while acting as a return path during the charging of its coupled stage.
There have been significant improvements to rectifier and capacitor technology, the above voltage multiplier suffers from a variety of issues including, for example, low reliability, high cost, and large size.
With regard to reliability, the above voltage multiplier typically has a poor thermal path for dissipation emanating from the diode in the center of series packaged HV diode assemblies. Further, initial voltage gradients are often sufficient enough to result in electron impact ionization and avalanche current.
Regarding the size issues, such voltage multiplier typically have coupling capacitors which must handle bidirectional currents sufficient to handle all stages above. Furthermore, bulky insulation is typically required in order to allow for an adequate lifetime under severe voltage stresses.
Such voltage multipliers are relatively expensive due to the special high voltage capacitors and diodes used.
Another approach to producing a high voltage output is by using multiple secondaries. Each of the secondaries produces rectified DC outputs, and each of the secondaries are series connected to provide a very high voltage. The secondaries may be arranged such that the AC fields between nearby secondaries are minimized, i.e., make the stresses DC, a much lower reflected capacitance is achieved than using a single secondary. This approach typically uses more relatively common available components and has a relatively improved diode thermal issue than compared to the above voltage multiplier. However, this approach has a ground referenced magnetic core which couples magnetic energy to the secondaries. This requires a significant amount of insulation because of capacitive coupling as compared to the above voltage multiplier.